There is considerable interest in using rate earth doped fiber amplifiers to amplify weak optical signals for both local and trunk optical telecommunications networks. The rare earth doped optical amplifying fibers are found to be low in cost, exhibit low-noise, have a relatively large gain bandwidth which is not polarization dependent, exhibit substantially reduced crosstalk problems and display low insertion losses at the relevant operating wavelengths, for example, approximately 1.55 .mu.m which are used in optical communications. A rare earth doped optical fiber amplifier can be coupled end-to-end to a transmission fiber and a laser diode pump, through an optical multiplexer. The optical multiplexer is designed to combine the signal which is to be amplified and the output from the laser diode pump with low loss. When the amplifying medium is excited with the optical power from the pump laser, signal light traversing the amplifier experiences gain. The pump energy may be made to propagate either co-directionally or contra-directionally relative to the signal energy, the direction of travel of the signal from the pump depending upon the noise requirements of the amplifier and whether any remaining unconverted pump light can be more conveniently filtered at the receiving end or transmitting end of the optical amplifier.
A complicating factor in the design of rare earth doped optical amplifiers involves the difference between the various parameters necessary to optimize the performance of the amplifier and those necessary to optimize the performance of the transmission system from end-to-end. In a transmission fiber, the spacing between repeaters, can be increased by minimizing loss in the fiber to reduce optical power requirements and by minimizing the fiber dispersion. However, in the amplifying fiber, as opposed to the transmission fiber, the major concern involves high gain, high saturation power and low noise, all with minimal pump powers. Additionally, because the signal mode size between the two fibers can be significantly different, splicing losses due to mode mismatch of the two fibers might be significant.
Currently, Erbium-doped fiber amplifiers appear to have the greatest potential for the high amplification necessary to overcome losses in the signal path including those of the various optical elements associated with the optical amplifier. Erbium-doped fiber amplifiers operate at .lambda.=1.53-1.56 .mu.m which is of particular interest for optical communication systems because, in this wavelength region, the amplifiers exhibit low insertion loss, broad gain bandwidth (approximately 30 nm) and polarization insensitive gain. Such amplifiers, for example, when pumped at .lambda.=1.48 .mu.m can have a gain as high as 35 dB but require as much as 70 mW of launched pump power. A higher gain together with a lower value of pump power is preferred.